Summary:
We extend -Wall with incomplete-uni-patterns and
incomplete-record-updates as per GHC proposal #12[1].

However, doing so gives rise to warnings due to code that is considered
unsafe per these two flags. For now, we just suppress these warnings
to make the build less noisy. The proper solution would be to refine
things so that these warnings go away. This idea is tracked by Trac #16041.

[1] https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0012-Wall-uni-patterns.rst

[skip ci]

Test Plan: make test TEST=T15656

Reviewers: RyanGlScott, bgamari

Subscribers: rwbarton, carter

GHC Trac Issues: #15656

Differential Revision: https://phabricator.haskell.org/D5415